How to sync Google and Outlook calendars (and keep the details private)

There are four practical ways to keep a Google and an Outlook calendar in step: subscribe one to the other with a secret iCal link (free, one-way, slow to refresh), copy events across by hand (reliable, tedious), federate free/busy at the organisation level (only if you administer both tenants), or use a sync tool that copies events automatically. The trade-off nobody spells out is privacy: the two free methods copy your full event details — titles, attendees, locations — onto the other account. If one of those accounts belongs to your employer, your personal event titles now live on their servers.

Option 1: Subscribe via secret iCal address (free, one-way)

Google Calendar publishes a private iCal feed you can subscribe to from Outlook. In Google Calendar, open Settings → your calendar → Integrate calendar and copy the Secret address in iCal format. In Outlook on the web, go to Add calendar → Subscribe from web and paste it.

Three things to know before relying on it:

• It is one-way. Google events appear in Outlook; Outlook events never travel back. To get both directions you repeat the process in reverse, which means two subscriptions to maintain.
• Refresh is on Microsoft's schedule, not yours. Subscribed internet calendars typically update every few hours, and can lag longer. A meeting you accept this morning may not show up in Outlook before someone books over it.
• It copies everything. Titles, descriptions, attendees and locations all cross over. Anyone with access to the receiving account — an IT administrator, a delegate, a shared-mailbox colleague — can read them.

Treat that secret address like a password. Anyone who gets the URL can read the whole calendar, and the only way to revoke it is to reset it in Google, which breaks every subscription using it.

Option 2: Copy events by hand (free, exact, tedious)

For a handful of recurring commitments, manually creating a matching “Busy” block on the other calendar is genuinely fine — and it is the only method where you control exactly what the other side says. Many people keep a single repeating block (“Unavailable”) rather than mirroring individual events.

It stops being fine the moment your schedule changes often. Every reschedule is now two edits, and the failure mode is silent: you move the original, forget the copy, and get double-booked against a meeting that no longer exists.

Option 3: Organisation-level free/busy federation (admins only)

Google Workspace and Microsoft 365 can be configured to exchange free/busy information directly, so users in one tenant see availability in the other without any event content crossing over. It is the cleanest option when it applies.

It rarely applies. You need administrator access to both tenants, which rules out the common case entirely: a work account you don't control plus a personal account, or several clients' accounts belonging to organisations that have never heard of each other.

Option 4: A sync tool (automated, and you choose what crosses)

Dedicated tools connect to both calendars and keep them in step continuously, usually within a minute or two, in both directions. The feature that matters most is the one that is easiest to overlook: what gets written onto the other calendar. A good tool lets you mirror a busy block with no details at all, so your work calendar shows when you are unavailable and never why.

Options in this space include OneCal, CalendarBridge, SyncThemCalendars and BusyBoard. They differ mostly in whether they also do anything with the merged result — sync-only tools stop once the calendars match.

BusyBoard is the automated way here. Each mirror rule has a display mode you choose: a bare Busy block with nothing else, the title only, or full details — so a work calendar can know you are unavailable without learning why. It then turns the merged picture into a link you can send, so people can see when you are free and book a time without ever seeing your events.

Which one should you use?

• Two calendars you own, details not sensitive: the iCal subscription is free and good enough, as long as you can live with the refresh lag.
• A work calendar you don't control:avoid anything that copies details onto it. Mirror busy-only blocks, or add a manual “Unavailable” block.
• Several clients, frequent changes: manual copying will fail eventually. This is what sync tools are for.
• You administer both tenants: use free/busy federation and skip third-party tools entirely.

The question behind the question

Most people looking for calendar sync don't actually want two identical calendars. They want to stop being double-booked, and they want to be able to tell someone when they are free without opening both calendars and doing the arithmetic. Syncing is one way to get there; merging your calendars into a single availability view and sharing that instead gets you the same outcome without duplicating anything.